1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus and, more particularly, to an optical scanning device, including a pickup for an optical disk or an optical device such as an optical switch for optical communication and an optical computer, for deflecting each light beam incident on an thin film optical waveguide by distributed refractive index of the optical waveguide, and an image forming apparatus such as a laser printer, a digital copying machine, or a facsimile using an optical scanning device.
2. Description of the Related Art
An optical scanning device, which is typically used in a laser beam printer, a digital copying machine, a facsimile, or the like, comprises a rotating polygon mirror for deflecting a light beam from a gas laser or a semiconductor laser and an f.theta. lens for focusing a light beam reflected from the rotating polygon mirror on an image forming surface such as a photosensitive member in the state of a uniform linear motion. In the optical scanning device using such a polygon mirror, since the polygon mirror is rotated at a high speed by a driving motor, durability is poor, and noise is generated in high-speed rotation. In consideration of the durability and noise, an optical scanning rate is disadvantageously restricted by the rotating speed of the driving motor.
For this reason, a light deflecting device serving as an optical waveguide type scanning device using an acousto-optic effect is proposed (C.S. Tsai, IEEE Trans. Circuits and Syst. vol. CAS-26 (1979) 1072., see Japanese Patent Application Laid-Open No. 52-68307, Japanese Patent Application Publication No. 63-64765, or the like). This optical waveguide type light deflecting device comprises an optical waveguide consisting of LiNbO.sub.3, ZnO, or the like, an directing means for coupling (directing) a light beam into the optical waveguide, a comb-like electrode (transducer) for exciting a surface acoustic wave to deflect the light beam in the light waveguide by an acousto-optic effect, and an emission means for emitting the deflected optical beam from the optical waveguide. In order to effectively emit the optical beam, a thin-film lens or the like may be added to the light deflecting device as needed. An optical waveguide type light deflecting device having no mechanically mobile member such as a polygon mirror is advantageous because the light deflecting device is free from noise and has high reliability and a small size.
In an image forming apparatus with such light deflecting device, deflection of a light beam by the light deflecting device is defined as main scanning, and movement in a direction perpendicular to the main scanning direction on a projection surface of a photosensitive member or the like irradiated with the light beam is defined as sub-scanning. In this case, scan lines formed by the main scanning on the projection surface can form an image surface which continues in the sub-scanning direction. By changing the intensity of the light beam according to image data depending on the density of an image or the like synchronously with the scanning (main scanning and sub-scanning), the image can be formed. In recent years, it is demanded that an output such as a high-quality print is formed within a short period of time, i.e., an increase in scanning rate is demanded.
However, disadvantageously, an acousto-optic light deflecting device generally has a limited laser scanning rate with an upper limit value due to a deflection rate limit. More specifically, when high-speed scanning is performed by one light beam deflected by the acousto-optic light deflecting device, deflection is performed by an excited surface acoustic wave. For this reason, a deflection rate is limited to a specific value, and a scanning rate has an upper limit value. Therefore, application of the light deflecting device for an image forming apparatus such as a laser printer, a digital copying machine, or a facsimile is limited.
In order to solve the above problem, as another light deflecting device having no mechanical mobile member, a prism type light deflecting device using a material having an electro-optic effect whose modulation rate higher than that of the acousto-optic effect is known (see A. Yariv, Optical Electronics, 4th ed. (New York, Rinehart and Winston, 1991) pp. 336 to 339, Q, Chen, et al., J. Lightwave Tech. vol. 12 (1994) 1401, Japanese Patent Application Laid-Open No. 62-47627, or the like). While the acousto-optic light deflecting device deflects a light beam by an excited surface acoustic wave, the electro-optic light deflecting device applies a signal to an electrode arranged in an optical waveguide to periodically distribute the refractive index of the optical waveguide, and a light beam is deflected by the distributed refractive index. As such a prism type light deflecting device, a bulk device using a ceramic or a single-crystal materials is known. However, since the bulk device itself is large in dimension, and is driven by a high voltage, a practical deflection angle cannot be obtained in the bulk device. Furthermore, a prism type domain inversion light deflecting device or a prism type electrode light deflecting device using an LiNbO.sub.3 single-crystal wafer in which a Ti-diffusion type optical waveguide or a proton-exchange type optical waveguide is formed is also known.
However, in order to obtain an electro-optic device, an electrode interval of about 5 mm which is equal to the thickness of the LiNbO.sub.3 single-crystal wafer is required. For this reason, a driving voltage for this type device becomes high. In the above prior art, a deflection angle of only about 0.5.degree. can be obtained at a drive voltage of .+-.600 V, and a practical deflection angle cannot be obtained.
In order to solve the above problem, the applicants has proposed an optical scanning device which has an optical waveguide, a light source for causing a light beam to be incident into the optical waveguide, and an electrode for deflecting the light beam in the optical waveguide by an electro-optic effect (Japanese Patent Application No. 7-176939).
Such an electro-optic optical scanning device does not have a problem associated with the scanning rate upper limit value because the electro-optic switching speed is high. However, a modulation rate of a driver for modulating the intensity of a light beam incident on the optical waveguide according to image data is limited to a specific value, and the scanning rate is limited to a specific value due to the limited modulation rate.
In order to solve the above problem, by scanning a plurality of light beams simultaneously or by scanning light beams determined selectively from a plurality of light beams, an modulation rate can be substantially increased.
However, in order to simultaneously scan a plurality of light beams, optical axes adjustment for positioning respective optical scanning devices corresponding to a plurality of light beams must be performed at a high accuracy to keep the scan-line interval of several tens microns so as to prevent ununiformity of the image surface. Optical axis adjustment for realizing the positional accuracy described above can be performed experimentally. However, since fine adjustment requires a long period of time and it must be performed in a complex structure, this method is not practical.
The present invention has been made in consideration of the above circumstances.
An object of the present invention is to obtain an optical scanning device which has a simple arrangement and can deflect a plurality of light beams while keeping a predetermined interval between a plurality of scan lines formed by the plurality of light beams is kept.
Another object of the present invention is to obtain an image forming apparatus using an optical scanning device.